> On Nov 12, 2019, at 1:21 PM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: > > CVE-2018-12207 is a microarchitectural implementation issue > that could allow an unprivileged local attacker to cause system wide > denial-of-service condition. > > Privileged software may change the page size (ex. 4KB, 2MB, 1GB) in the > paging structures, without following such paging structure changes with > invalidation of the TLB entries corresponding to the changed pages. In > this case, the attacker could invoke instruction fetch, which will result > in the processor hitting multiple TLB entries, reporting a machine check > error exception, and ultimately hanging the system. > > The attached patches mitigate the vulnerability by making huge pages > non-executable. The processor will not be able to execute an instruction > residing in a large page (ie. 2MB, 1GB, etc.) without causing a trap into > the host kernel/hypervisor; KVM will then break the large page into 4KB > pages and gives executable permission to 4KB pages. It sounds that this mitigation will trigger the “page fracturing” problem I once encountered [1], causing frequent full TLB flushes when invlpg runs. I wonder if VMs would benefit in performance from changing /sys/kernel/debug/x86/tlb_single_page_flush_ceiling to zero. On a different note - I am not sure I fully understand the exact scenario. Any chance of getting a kvm-unit-test for this case? [1] https://patchwork.kernel.org/patch/9099311/
